Stretching installations are used in particular for the production of plastics films. So-called simultaneous stretching installations are known in which a plastics film can be stretched simultaneously in a transverse direction and in a longitudinal direction. Likewise, sequential stretching installations are known in which a plastics film is stretched in two successive stages, for example firstly in a longitudinal direction and then in a transverse direction (or vice versa).
An already-known transverse stretching installation or transverse stretching stage within a stretching installation is known for example from U.S. Pat. No. 5,797,172 A. In said prior publication, a material web to be stretched, that is to say generally a plastics film, is taken hold of by way of clips, which are fastened to chains and which are arranged, on both sides of the material web to be stretched, on in each case one circulating guide path. The clips are in this case moved in succession from a run-in zone (in which the edge of, for example, a plastics film to be stretched is taken hold of) via a stretching zone (in which the opposite clips on the guide rail sections are moved away from one another with a transverse component diverging from the transport direction) to a run-out zone, and then, on a return path, back to the run-in zone, wherein, in the run-out zone, the film can undergo for example a final relaxation and/or heat treatment process.
In this case, the clips are composed of a so-called clip transport unit which comprises firstly the clip part itself and secondly the so-called transport part, that is to say the clip device and the transport device. In the already-known prior art according to U.S. Pat. No. 5,797,172 A, the so-called transport part is ultimately a chain part, as the clips for the discussed transverse stretching installation are connected to one another by way of corresponding chain links.
According to said previously published prior art, the clip transport unit is in this case supported by way of slide elements on two opposite sides of a guide rail, on the one hand, and on a support rail provided below the guide rail, on the other hand.
Instead of slide elements of said type, it is however likewise also possible to use roller elements in order to permit movement of the clip transport unit, supported for example on a guide rail and on a weight-sustaining running rail. This is known for example from DE 39 28 454 A1. Said document describes a guide rail in the form of a so-called monorail, which guide rail has a square cross section. In this case, the clip transport unit is supported by way of running wheels, so-called rollers, which roll on the top side and on the bottom side and on the two vertical sides situated offset in a horizontal direction, whereby the clip transport unit can be moved along said guide rail. A clip transport unit of said type is likewise suitable in particular for a stretching frame, that is to say a transverse stretching installation.
In the case of the already-known stretching installations, there is the basic problem of ensuring that the friction coefficients for the rolling and/or sliding friction do not become too high. This is because the acting friction makes it necessary for lubricant, in particular oil, to be used in order to reduce the friction. Here, it is pointed out that not only does the friction contribute to considerable power losses, but also the power losses arise, in particular in the case of friction bearing arrangements, in the form of friction losses, that is to say in particular the power lost is released in the form of heat to the guide system. At high speeds, it is therefore necessary for conventional slide guides to be cooled in order to prevent decomposition (cracking) of the lubricating oil.
By contrast, it is an object of the present invention to provide an improved transport system, in particular in the form of a transverse stretching installation.
The object is achieved according to the invention in accordance with the features specified in claim 1. Advantageous refinements of the invention are specified in the subclaims.
In the context of the present invention, a transport system is provided for transport chains equipped, for example, with clips and which (aside from sprockets) are movable in rail-bound fashion along a guide path. In the case of the support rails provided in this transport system, by contrast to conventional solutions, no oil lubrication is realized for the purposes of reducing the friction or sliding coefficients, and instead, here, it is provided that an air bearing in the form of an air cushion is formed.
Air bearings of said type exhibit an extremely low friction coefficient. The friction coefficient is however a crucial factor in the case of such sliding transport systems. Said friction coefficient determines inter alia the dimensioning of the chain. In the case of a low friction coefficient, the chain longitudinal force decreases, whereby the chain can be configured for lower loads. Thus, the chain weight is also reduced, whereby in turn, the chain longitudinal force is reduced. A lower friction coefficient thus also permits the formation of a more lightweight transport chain. As has already been discussed in conjunction with already-known transport systems, in particular of transverse stretching installations, the friction basically gives rise to considerable power losses. The power lost is released, through friction losses, in the form of heat to the guide system. This is the reason why, in the presence of high installation speeds, conventional slide guides must be cooled in order to prevent the abovementioned decomposition (cracking) of the lubricating oil film. In the case of the air bearing-mounted guide system provided in the context of the present invention, there is no risk of this because, firstly, large amounts of heat are not generated, and secondly, no oil is used either.
A further major advantage arises from the fact that, in the context of the present invention, it is not necessary to use oil as lubricant. This is because the oil lubrication in the case of conventional transport systems, in particular stretching installations, harbors the inherent risk of the oil film that is used leading to fouling of the film, which should and must be prevented in all situations, for which reason cumbersome structural measures are required for this purpose in conventional installations. Since, as mentioned, oil is not used in the case of air bearing-mounted systems, an oil shielding arrangement for preventing oil fouling is also not required.
The discussed reduction in power losses furthermore has a positive effect on the required drive power. This is because, in the context of the air bearing arrangement provided according to the invention, it is now also possible for use to be made of smaller drive motors.
It finally remains to be stated that conventional transport sliding systems with a conventional oil lubrication arrangement ultimately also reach their physical limits at high speeds. By contrast, very much higher installation speeds can be reached with the air bearing-mounted transport system provided according to the invention.
Air bearing technology is itself fundamentally known. However, air bearing technology is presently used primarily for measurement machines and for low-speed applications. Furthermore, such applications are normally found in clean environments.
Furthermore, the use of air bearing technology for moving extremely heavy equipment is also known, wherein conventionally, the equipment to be moved serves, itself, for generating the air cushion. In the case of such vehicles and equipment operated using air cushions, even a contaminated environment generally does not constitute a problem. However, in this case, too, it is normally the case that only low speeds are reached, and it is normally also the case that only relatively short distances are covered. Here, it is normally the case, in standard applications, that the air bearing itself is supplied with air.
Furthermore, it is basically also known for air bearing technology to be used in a circulating transport system. In this regard, reference is made to DE 2 149 032 A, in which a conveyor belt is guided on an encircling path in the form of a support body. Between the underside of the encircling conveyor belt and the support body, there is provided an air bearing which bears the conveyor belt. The air bearing is in this case supplied with compressed air through openings in that surface of the support body which lies below the conveyor belt. For this purpose, the support body has, in cross section, a rectangular shape, in the interior of which there is formed a compressed-air duct, such that the compressed air supplied here can then flow out through the openings, situated on the top, in the direction of the underside of the conveyor belt and thus bear the conveyor belt.
To keep the air consumption low, it is furthermore the case that lateral sealing lips are provided. In the case of an already-known transport system of said type, however, it is merely intended for a more or less flat transport belt to be moved along on the air cushion that is formed, that is to say an areal structure with a relatively large width extent and, in relation thereto, only a minimal height.